1. Field of the Invention
The present invention relates to a variable valve control device for changing opening/closing timing and lift stroke of at least one of an intake valve and an exhaust valve for an internal combustion engine (hereinafter referred to simply as an "engine") according to engine operating conditions.
2. Description of Related Art
There has been known a vane type variable valve control device for controlling opening/closing timing of at least one of the intake and exhaust valves in a manner that a camshaft is driven via a timing pulley or a chain sprocket which rotates in synchronism with an engine crank shaft with a phase difference by relative rotation between the timing pulley or the chain sprocket and the camshaft. Such a variable valve control device is operative for changing a mutually overlapping valve-open period of the intake and exhaust valves to secure more stable engine operation, less fuel consumption and lower exhaust emission.
Further, as disclosed in JP-A-9-32519, there is also a known variable valve control device in which the valve-open period and the lift stroke of at least one of the intake valve and the exhaust valve are changed by axially moving a camshaft provided with a cam having an axially different profile.
Furthermore, according to the variable valve control device shown in JP-A-9-32519, the valve-open period and the lift stroke is changed by controlling the pressure to a hydraulic actuator according to engine operating conditions so that more stable engine operation, less fuel consumption and lower exhaust emission may be further secured.
However, the conventional variable valve control device, in which phase control means for adjusting the angular phase of the camshaft relative to the crankshaft and axial movement control means for changing the valve-open period and the lift stroke of one of the intake and exhaust valves are combined and controlled independently of each other, has the following problems.
At first, in the cam having an axially different profile, a camshaft receives a thrust force acting toward a lower lift stroke position due to an axially tapered cam profile. When a movable piston as a hydraulic actuator is held at the lowest lift stroke position, the movable piston is in contact with and is pushed onto an axial end surface of a pressure chamber at the lowest lift stroke position by the thrust force. Therefore, a responsiveness of the angular phase control is adversely affected by a friction between the piston and the axial end surface.
Secondly, when the movable piston is held at the highest lift stroke position by supplying sufficient oil to a high lift side pressure chamber, the movable piston is in contact with and is pushed onto the other axial end surface restricting the cam shaft stroke. Thus, a friction between the piston and the other axial end surface causes a worse responsiveness of the angular phase control.
Thirdly, when the movable piston is at the lowest lift stroke position and air is invaded into the high lift side pressure chamber or when the movable piston is at the highest lift stroke position and air is invaded into the low lift side pressure chamber, a responsiveness of shifting the movable piston from the lowest lift stroke position toward the higher lift stroke position or from the highest lift stroke position toward the lower lift stroke position is adversely affected.